There is no detail around the requirements for obtaining this rare Shiny Legendary Beasts for Pokemon GO but judging from how hard it is it to obtain just a regular shiny Pokemon, don’t be surprised to see these tied to some specific requirement aside from being completely random.

You are able to capture the newly added Johto Legendary Pokemon starting August 31st. Details on how to capture them can be read from here. The addition of the Shiny Legendary Pokemon is only seen in the form of some in-game asset files as seen in the image below.

Legendary Pokemon have started to appear from this Summer and the game developer Niantic Labs has slowly made available more of the Legendary Pokemon after a certain time. MewTwo was also available to capture in Pokemon GO but it was limited to a very specific location and considered one of the rarest Pokemon currently available to capture in Pokemon GO, but Niantic Labs has plans to bring it to more locations around the world.